A transmission to be mounted in a vehicle such as an automatic transmission is configured such that gearshift is automatically performed by selective engagement of a power transmission path constituted by a planetary gear mechanism and the like with a plurality of hydraulic friction engagement elements. Each gear position is basically formed by engagement of two friction engagement elements. Conventionally, regarding a first gear position in the D-range, it is a usual practice to form the gear position by one friction engagement element and an OWC (one-way clutch) for the purpose of a smooth gearshift operation and the like.
However, the OWC costs high. Further, a rotational resistance may be generated in a gear position other than the first gear position in the D-range. This may obstruct improvement of fuel economy of an engine. In view of the above, in recent years, abolishing use of the OWC is proposed or executed.
For instance, the first gear position is formed by engagement of a friction engagement element such as a low clutch, which is engaged with a predetermined low speed gear positions including the first gear position, and a friction engagement element such as a low reverse brake, which is engaged with the first gear position and a reverse gear position. Gearshift to the first gear position is performed by engagement of the latter friction engagement element in a state that the former friction engagement element is engaged. Therefore, in order to smoothly shift from another gear position to the first gear position, it is necessary to finely control the timing or the engaging force in engaging a friction engagement element such as the low reverse brake.
In order to cope with the aforementioned problem, Patent Literature 1 discloses a low reverse brake using a tandem hydraulic actuator including two pistons.
As illustrated in FIG. 13A and FIG. 13B, a brake device 100 is configured such that a plurality of fixed-side friction plates 103 that are spline-engaged with an inner surface of a transmission case, and a plurality of rotation-side friction plates 104 that are spline-engaged with an outer surface of a rotary member are alternately disposed between a transmission case 101, and a rotary member 102 accommodated in the case 101; and the brake device is provided with an engaging piston 106 for engaging the friction plates 103 and 104 against a biasing force of a return spring 105, and a clearance adjustment piston 107 disposed behind the piston 106.
In the brake device 100, as illustrated in FIG. 13A, at the time of non-engagement, the pistons 106 and 107 are held at a retracted position by a biasing force of the return spring 105, and a relatively large clutch clearance (a value obtained by subtracting the sum of the thicknesses of all the friction plates from a distance between the engaging piston and a retaining plate) is formed.
When a hydraulic pressure is supplied to a hydraulic chamber 109 of the clearance adjustment piston 107 (hereinafter, referred to as a “clearance adjustment hydraulic chamber”) from the aforementioned state, as illustrated in FIG. 13B, the pistons 106 and 107 are advanced to a stroke end of the clearance adjustment piston 107 against a biasing force of the return spring 105. Thereby, the clutch clearance is reduced by a distance by which the pistons 106 and 107 are advanced.
Therefore, by supplying a hydraulic pressure to the clearance adjustment hydraulic chamber 109, it is possible to engage the friction plates 103 and 104 with enhanced responsiveness when a hydraulic pressure is supplied to a hydraulic chamber 110 of the engaging piston 106 (hereinafter, referred to as an “engagement hydraulic chamber”). Thus, it is possible to finely control the timing or the engaging force of an engaging operation. On the other hand, at the time of release, namely, when a hydraulic pressure is not supplied to the hydraulic chambers 109 and 110, and the pistons 106 and 107 are held at a retracted position by a biasing force of the return spring 105, a relatively large clutch clearance is formed. Therefore, a sliding resistance between the fixed-side friction plates 103 and the rotation-side friction plates 104, namely, a resistance of a rotary member is small. This makes it possible to improve the fuel economy of an engine, and to suppress an increase in electric power consumption of a motor.
However, in the brake device described in Patent Literature 1, at the time of non-engagement, a gap between each adjacent pair of the friction plates may vary, and a sliding resistance between the friction plates may increase at a portion where the gap is small. An increase in the sliding resistance between the friction plates may lead to drive loss increase in a transmission as a whole, and may deteriorate the fuel economy.